This invention relates in general to an improvement in a system and method for controlling exhaust gas recirculation (EGR) for an internal combustion engine of a motor vehicle, and more particularly to an improvement in an EGR control system of the type having an EGR control valve controlling communication between an exhaust gas conduit and an intake air conduit, a constant-vacuum valve for applying a vacuum to the EGR control valve, a pulse-operated electromagnetic valve for regulating the vacuum applied to the EGR valve, and a control unit for controlling the opening and closing of the electromagnetic valve in response to signals indicating an operating condition of the vehicle, and so arranged that the control unit gives the electromagnetic valve a command to open the EGR control valve such that an opening degree for exhaust gas recirculation in a required value is obtained, when the operating condition of the vehicle reaches the predetermined level.
In an EGR control system of the above-mentioned type when, for example, vehicle speed and the opening degree of a throttle valve for controlling intake air amount are below predetermined levels, respectively, exhaust gas recirculation is not, generally, carried out. When the vehicle speed and the throttle valve opening degree exceed the predetermined levels, respectively, the vacuum applied from the constant-vacuum valve to the EGR control valve is controlled in response to a pulse width by which fuel injection amount is controlled, which pulse width corresponding to engine speed and intake air amount. The thus controlled vacuum operates the EGR control valve so as to obtain the exhaust gas recirculation in a required value. However, such an EGR control system has encountered a problem in that a relatively long period of time is necessary until a required opening degree of the EGR control valve is actually obtained from a time point at which the opening degree of the throttle valve reaches the predetermined level upon a vehicle speed higher than the predetermined level. As a result, the starting of the exhaust gas recirculation is delayed and accordingly exhaust gas recirculation cannot be performed when required.